1. Field of the Invention
The present invention relates to a system and method for managing water content in a fluid.
2. Background Art
Conventionally, water is collected from air, or other gaseous fluids, using condensation systems. An exemplary condensation system provides a surface cooled to a temperature that is at or below the dew point of incoming air. As is well known in the art, the cooling of air at or below its dew point causes the condensation of water vapor from the air and a decrease in the absolute humidity of the air. The humidity of a volume of air is substantially determinative of the amount of water that can be introduced into, or removed from, the volume of air.
The humidity and temperature of air varies, however, from region to region, with hot and humid air in tropical and semi-tropical regions, and cooler, less humid air in other parts of the world. The temperature and water vapor content of air also varies widely with seasonal weather changes in regions throughout the year. Therefore, depending on the region of the world, and depending on the time of year, humidification or dehumidification may be desirable, for example, to make an environment more comfortable.
In addition to increasing comfort, management of the amount of water in air may be important to industrial applications. Moreover, it may be desirable to remove water from air so that the water can be utilized, for example, for drinking, or in other applications where fresh water is desired. Regardless of the reason for managing the amount of water in the air, there are times when conventional water management systems have undesirable limitations. For example, when the dew point of the air is low, particularly when it is below the freezing point of water, it may be difficult or impossible to remove the water using a condensation system. One way to remove water from air even when the dew point is low is to use a system utilizing a desiccant to extract water from the air.
In a desiccant system, both heat and mass are transferred to and from the air. Conventional systems of this type are generally inefficient in at least one of the two types of transfer—i.e., heat or mass transfer—because the transfer of one inherently transfers the other, which may be undesirable. For example, a desiccant wheel can be used to remove water vapor from an airflow, thereby transferring mass out of the air and reducing the enthalpy of the air. At the same time, however, a large amount of heat may be added by the phase change occurring as the water condenses out of the air; this causes an increase in the enthalpy of the air.
Conventional desiccant based dehumidifiers generally require the movement of the desiccant from a first region where it absorbs moisture—i.e., a “collection” or “dehumidfying” station—to a second region where it expels the moisture—i.e., a regeneration station. In the case of solid desiccants, this transfer is achieved by physically moving the desiccant from a dehumidifying station to a regeneration station, for example, by mounting the desiccant on a rotating wheel, a belt or the like. In liquid desiccant systems, two pumps are generally provided: one for pumping the liquid to the regeneration station, and the other for pumping the liquid from the regeneration station to the dehumidifying station. In some embodiments, a single pump is used to pump from one station to the other, with the return flow being gravity fed.
One such system removes air from a first airflow by spraying the first airflow with a liquid desiccant. The desiccant may be cooled prior to being sprayed. Water removed from the air is collected by the desiccant, which becomes increasingly diluted. The cool, diluted desiccant is collected in the bottom of a collection chamber. On the other side of the system, the diluted desiccant is heated and brought into contact with a second airflow, which removes the water from the desiccant, thereby leaving it more concentrated. The warm, concentrated desiccant is collected in the bottom of a regeneration chamber.
The two chambers may be connected, for example by an orifice, to allow mixing of the diluted and concentrated desiccant pools. Because a concentration gradient will exist between the diluted and concentrated desiccants, diffusion between the two chambers will naturally occur. Although the orifice may be an efficient mechanism to transfer mass—i.e., the water ions—it also facilitates heat transfer as the warm, concentrated desiccant mixes with the cool, diluted desiccant. This may be acceptable in some applications, but in others, it may be desirable to have a system that controls both heat and mass transfer.
Another type of air conditioning desiccant system is described in U.S. Pat. No. 4,941,324 issued to Peterson et al. on 17 Jul. 1990. Peterson et al. describes a mechanism to transfer liquid desiccant between a condenser sump and an evaporator sump. Dilute desiccant from the evaporator sump is transferred into the condenser sump, and concentrated desiccant from the condenser sump is transferred back to the evaporator sump. The transfer mechanism includes a pair of pumps and a series of globe valves that control the amount of desiccant transferred between the sumps and the amount of desiccant delivered to desiccant distributors.
One limitation of the Peterson et al. system is limited control over the amount of desiccant transferred between the sumps. Specifically, such a system may result in undesirably large quantities of desiccant being pumped between the two sumps in order to continuously regenerate the desiccant. Because the temperature of the desiccant in the condenser sump may be significantly higher than the temperature of the desiccant in the evaporator sump, an undesirable amount of heat transfer can occur as the large mass of liquid is transferred between the sumps. This can be very inefficient. To help reduce this inefficiency, the Peterson et al. system utilizes a heat exchanger to transfer heat between the two desiccant streams as they are pumped between the two sumps. Although this may reduce some of the inefficiency, the process may yet be undesirably inefficient because of the large quantity of liquid being transferred.
In many different fields—e.g., air conditioning, collecting water from air, and generating power using a combustion engine or gas turbine—controlling the transfer of both heat and mass of one or more materials is important to the overall efficiency of the process. Therefore, there is a need for a system and method for managing the water content in a fluid that can extract water from the fluid under a variety of ambient conditions utilizing a desiccant that is at least partly liquid, and that can efficiently control the transfer of both mass and heat of the water to and from the desiccant.